The present embodiments relate to a method for controlling a magnetic resonance system having a plurality of radio-frequency transmit channels.
In a magnetic resonance system or magnetic resonance tomography system, the body to be examined may be exposed to a relatively high basic magnetic field (e.g., the “B0 field”) of 3 or 7 tesla, for example, with the aid of a basic field magnet system. A gradient system is also used to apply a magnetic field gradient. Radio-frequency excitation signals (RF signals) are emitted via a radio-frequency transmit system using suitable antenna facilities with the aim of tipping the nuclear spins of certain atoms that have been excited in a resonant manner by this radio-frequency field (e.g., the “B1 field”) with spatial resolution through a defined flip angle in relation to the magnetic field lines of the basic magnetic field. This radio-frequency excitation or the resulting flip angle distribution are hereinafter also referred to as nuclear magnetization or “magnetization”. The relationship between the magnetization m and the B1 field emitted over a period T is obtained according to
                    m        ≈                  2          ⁢                                          ⁢          π          ⁢                                          ⁢                      γ            ·                                          ∫                                  t                  =                  0                                T                            ⁢                                                                    B                    1                                    ⁡                                      (                    t                    )                                                  ⁢                                                                  ⁢                                  ⅆ                  t                                                                                        (        1        )            
γ is the gyromagnetic moment, t the time variant, and B1 (t) is the time-varying magnetic field strength of the B1 field. On the relaxation of nuclear spins, radio-frequency signals (e.g., magnetic resonance signals) are emitted, received by suitable receive antennas, and further processed. The raw data acquired in this way may be used to reconstruct the desired image data. The emission of the radio-frequency signals for nuclear spin magnetization may take place by a “whole body coil” or “body coil” or also by of local coils placed on the patient or test subject. A typical structure of a whole body coil is a birdcage antenna including a plurality of transmit rods arranged parallel to the longitudinal axis around a patient chamber of the tomography system in which a patient is present during the examination. The end faces of the antenna rods are interconnected in a capacitive manner in a ring.
Whole body antennas may be operated in a “CP mode” (circularly polarized mode). A single temporal RF signal is sent to all components of the transmit antenna (e.g., all transmit rods of a birdcage antenna). In this case, the transfer of the pulses with identical amplitudes to the individual components may take place with a phase offset with a displacement matched to the geometry of the transmit coil. For example, in the case of a birdcage antenna with 16 rods, the rods are each activated with the same HF magnitude signal with 22.5° phase displacement. The result is a circularly polarized radio-frequency field in the x-/y plane (e.g., perpendicular to the longitudinal axis of the birdcage antenna extending in the z direction).
The radio-frequency signal may be modified to be emitted (e.g., the incoming sequence of radio-frequency pulses) individually with respect to amplitude and phase by a complex transmit scaling factor. In this case, the B1 field at a location r (e.g., at a pixel or voxel position r, where r is a vector with the values of the Cartesian coordinates x, y, z in mm) is represented by
                                          B            1                    ⁡                      (            t            )                          =                              ∑                          c              =              1                        N                    ⁢                                                    E                c                            ⁡                              (                r                )                                      ·                                          b                c                            ⁡                              (                t                )                                                                        (        2        )            
In this case, bc(t) is a RF curve to be transmitted on the channel c=1, . . . , N (e.g., the voltage amplitude distribution pattern (in V) of an RF pulse train over time t, which is specified by bc(t)=SFc·bR(t), where SFc is the complex scaling factor for the channel c, and bR(t) is the voltage distribution pattern of the reference pulse train). Ec(r) is the sensitivity (in μT/V) of the antenna element of the radio-frequency transmit channel c at a specific location r (e.g., the pixel or voxel position). Ec(r) is the position-dependent sensitivity distribution in the form of a sensitivity matrix.
The antenna may be operated in “CP mode” in that the amplitude is selected at the same level for all transmit channels, and a phase shift matched to the geometry of the transmit coil is provided. In addition, depending upon the object to be examined, an “EP mode” (elliptically polarized mode) with which the radio-frequency field in the x-/y plane is not circularly polarized, but elliptically polarized may be used. The mode used may be determined by the shape of the body area to be excited. In the case of objects that may be cylindrically symmetrical (e.g., in the case of images of the head region), the CP mode may be selected. In the case of more elliptical shapes (e.g., examinations in the thoracic or abdominal regions), the EP mode is chosen. The object of the EP mode is to compensate inhomogenities of the B1 field caused by non-circularly symmetrical body shapes. “B1 shimming” of a multi-channel radio-frequency transmit system may be performed. The individual transmit scaling factors are calculated on the basis of a patient-specific adjustment generally with the aim of calculating a particularly homogeneous excitation compared to the previous standard CP or EP mode.
In this case, the transmit scaling factors are calculated by optimizers that minimize the magnitude deviation of the perfectly homogeneously desired target magnetization m of the theoretically achieved actual magnetization A·b:b=argb min(∥A·b−m∥2)  (3)
A is the design matrix including a system of complex linear equations into which the spatial transmit profiles of the individual transmit channels (e.g., antenna rods) and the present B0 field distribution are inserted. This design matrix is, for example, described in W. Grissom et al.: “Spatial Domain Method for the Design of RF Pulses in Multicoil Parallel Excitation”, Mag. Res. Med. 56, 620-629, 2006. b(t) is the vector of the RF curves Mt) to be transmitted in parallel. If the solution to Equation (3) (e.g., the minimum of the “target function” defined in Equation (3)) is found, SF1, SF2, . . . , SFN are available as the result of the desired scaling factors.
In the case of a plurality of measurements or examinations, the “B1 performance” is a further criterion. In this case, the object is to achieve a specific target magnetization m as efficiently as possible (e.g., quickly) at a specific location (e.g., in a specific region of interest (ROI)). According to Equation (1), it is advantageous to achieve the highest possible B1 field. In order to increase the B1 field, according to Equation (2), the voltage distribution patterns bc may be increased (e.g., the voltage amplitudes of the RF pulses). Purely for technical reasons, this is not automatically possible since the individual components located in the transmit chain of the magnetic resonance system such as, for example, RF amplifiers, cables, measuring devices, and adapter networks are to be protected against overvoltage. Therefore, on the transmission of a pulse train, the pulse train is first checked with regard to the voltage compatibility with respect to the relevant components, and hence, the voltage of the pulse train Mt) is limited. For example, this problem occurs with B1 shimming since due to the complex scaling factors SF1, SF2, . . . , SFN on adjacent channels c, transmitted pulse trains Mt) are not only scaled in amplitude, but may also be phase-shifted with respect to each other, so that voltage differences that are higher than the maximum amplitude of the individual pulse trains Mt) may occur between these channels.